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Washington (CNN) -- The air traffic controller suspended for failing to respond to two planes heading into Ronald Reagan Washington National Airport has told investigators that he had fallen asleep, according to the National Transportation Safety Board.

The controller, a 20-year veteran, "indicated that he had fallen asleep for a period of time while on duty," according to a statement released Thursday by the safety board. "He had been working his fourth consecutive overnight shift (10:00 p.m. to 6:00 a.m.)."